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Heterotopic bone formation in soft tissues occurs commonly in Paget's disease patients following a primary total hip arthroplasty (THA). The nature of this heterotopic bone has not been documented. In this report, we show that the heterotopic bone removed 14 years after primary THA in a case of Paget's disease was sclerotic, contained prominent mosaic cement lines and showed increased remodelling activity on the bone surface. In addition to these typically Pagetic histological features, it was noted ultrastructurally that the osteoclasts contained characteristic intranuclear viral-like inclusions. In contrast, the foreign body macrophages found in the joint pseudocapsule and pseudomembrane, which are a population of mononuclear precursor cells from which osteoclasts can be formed, did not contain viral-like inclusions. These findings are of interest regarding the pathogenesis of heterotopic bone formation following hip arthroplasty and the ontogeny of Pagetic osteoclasts.